The Chemistry of CO2

Walter E. Cleland, Jr. Ph.D.Department of Chemistry and

Biochemistry25 February 2008

Outline

• Introduction• Physical Properties• Uses• Production, Recovery, Purification• Chemical Properties

Oxides of Carbon

• Stable Oxides of CarbonCOCO2

C3O2

C12O9

C O

C OO

C CO C O

O

O

O

O

OO

O

O

O

Selected Physical Properties of CO2

2349 cm-1 (Asym C=O), 1285-1388 cm-1 (Sym C=O; Raman), 667 cm-1 (Bending)1700 - 3000 Å

IR (gas)

UV (gas)

-394.4 kJ/mol-393.5 kJ/mol3.02 J/mol K531.4 kJ/mol

∆Gºf (CO2(g))∆Hºf (CO2(g))∆Sºf (CO2(g))Dissociation Energy (C-O)

1950 ppm (wt)392 x 10-4 mol/L atm

Solubility in water at 25oCHenry’s Law constant

1.823 Kg/m3

1.52Density of Gas @1atm and 21.1 0CRelative density of gas @1atm and 21.1oC (Air=1)

31°C72.85 atm (1070.67 psi)

Critical TCritical P

-56.6 °C (5.2 atm)-78.5 °C (subl)

MpBp @1atm

44.01g/molColorless, Odorless GasMolecular Weight

Phase Diagram

Walsh Diagram

Industrial Uses of CO2

Uses Based on Physical Properties (major)

• Refrigeration• Cleaning fluid• Air conditioning• Solvent for

ReactionsExtractionsNanopartical productionFood and Agro-chemical Applications

• Fumigant• Additive to beverages

Inert atmosphere

Uses Based on Chemical Properties

• Production of Urea• Production of Salicylic

acid• Inorganic carbonates and

pigments• Production of Propylene

carbonate• Neutralization of caustic

waste water

Production Sources

• Ammonia/Hydrogen plantsCH4 + 2 H2O → CO2 + 4 H2

• Flue gases from combustion• Fermentation

C6H12O6 → 2 C2H5OH + 2 CO2

• Lime-kiln operationsCaCO3 → CaO + CO2

• Sodium phosphate manufacture3 Na2CO3 + 2 H3PO4 → 2 Na3PO4 + 3 CO2 + 3 H2O

• Natural CO2 gas wells

Recovery by Reversible Absorption

• Sodium or Potassium Carbonate ProcessK2CO3(aq) + H2O(l) + CO2(g) KHCO3(aq)

• Girbotol Amine Process2 HOCH2CH2NH2(aq) + H2O(l) + CO2(g)

(HOCH2CH2NH3)2CO3(aq)

Purification

• Permanganate Process3 H2S + 2KMnO4 + 2 CO2 →

3 S + 2 MnO2 + 2 KHCO3 + 2 H2O

• Dichromate Process3 H2S + K2Cr2O7 + H2O + 2 CO2 →

3 S + 2 Cr(OH)3 + 2 KHCO3

Chemical Properties

• General Reactivity• Aqueous chemistry• Reactions of Industrial Importance• Biological Reactions• Organic Reactions• Coordination Chemistry• Reactions incorporating CO2 without Reduction• Reactions with Reduction

General Reactivity

• “Not very reactive at ordinary temperature”; “nonreactive”; “stable”; “inert”

• Many reactions of CO2 are thermodynamically favorable, but kinetically slow

• Ex: M2SiO4 + 2 CO2 → 2 MCO3 + SiO2

Aqueous Chemistry

• Species involved: CO2, H2CO3, HCO3-, CO3

2-, H+, OH-,[H2CO3

*] = [CO2(aq)] + [H2CO3]• Equilibrium concentrations described completely by system of 5

equations

CO2 + H2O H2CO3 K1 = 1.67 x 10-3

H2CO3* H+ + HCO3

- K2 = 4.47 x 10-7

H2CO3 H+ + HCO3- Ka = 2.68 x 10-4

HCO3- H+ + CO3

2- K3 =4.68 x 10-11

H2O H+ + OH- Kw = 1.0 x 10-14

CT = [H2CO3* ] + [HCO3

-] + [CO32-]

• Carbonates insolubleKsp(CaCO3) = 2.8 x 10-9; Ksp(MgCO3) = 3.5 x 10-8

Slow Kinetics for Reactions of CO2

• At pH < 8CO2 + H2O H2CO3 (slow)H2CO3 + OH- HCO3

- (fast)

• At pH > 10CO2 + OH- HCO3

- (slow)HCO3

- + OH- CO32- (fast)

• At pH 8-10 both sets of equilibria are important.

Important Reactions of CO2, CO and O2

At high temperature• 2 C + O2 → 2 CO ∆Hº = -221.0 kJ/mol• C + O2 → CO2 ∆Hº = -393.5 kJ/mol• C + H2O →CO + H2 ∆Hº = +131.3 kJ/mol• 2 CO C + CO2 ∆Hº = -172.5 kJ/mol• CO2 CO + 1/2 O2

Also• CO2 + H2 → CO + H2O (reverse of water-gas shift rxn)• CO2 + 2 NH3 → NH4O2CNH2 → NH2CONH2 + H2O

Basic Organic Reactions

Coordination Chemistry

• CO2 is a poor ligand• A number of

complexes and bonding modes known

• Important for activation of CO2 in reduction reactions

Reactions of “M-CO2”

• With Protons or other electrophile“M-CO2” + 2H+ + 2e- → “M-CO” + H2O“M-CO2” + R+ → M-C(O)OR

• Hydride insertion“M-H” + CO2 → “M-O2CH”

• With External Phosphine“M-CO2” + PR3 → “M-CO” + O=PR3

• With Coordinated Isonitrile“M(CNR)(CO2)” → RNCO + “M-CO”

Biological Reactions

• Ubiquitous in Nature• Animal metabolism

C6H6O6 + 6 O2 → 6 CO2 + 6 H2O + energy• Photosynthesis is reverse of above reaction• Numerous enzymes catalyze reactions of CO2

Carbonic anhydrase (Zn2+) - H2CO3 → CO2RuBisCO (Mg2+) - carboxylation of riboseCarbon monoxide dehydrogenase (Ni,Fe) - CO2 → CO

Use of CO2 as C1 Feedstock

• “Conversion” reactions (CO2 not reduced)

• Carboxylates, lactones RCOOR’

• Carbamates, RR’NCOOR”

• Ureas, RR’NCONRR’• Isocyanates, RNCO• Carbonates, ROC(O)OR’

• Reduction of CO2

• Formates, HCOO-

• Oxalates, -O2C-CO2-

• Formaldehyde, H2CO• Carbon Monoxide, CO• Methanol, CH3OH• Methane, CH4

Carboxylation Reactions

• C-C bond formation• Direct carbonylation in

ionic liquid from imidazoliumcarbonate

• N-C, O-C bond formation also known

Plastics from CO2

• Metal Salen complexes as catalysts

• Production ofPolycarbonates from Epoxides and CO2

CO2 Hydrogenation to form Formic Acid

Production of Methanol

• CO + 2 H2 → CH3OH• CO2 + 3 H2 → CH3OH

Cu-Zn-O catalyst, 250 - 300ºC, 5-10 MPaUp to 30% CO2 added to syngas feedstocksignificantly improves yield

• CO2 + H2O → CH3OHCascade of enzymesCO2 → HCOO- (formate dehydrogenase)HCOO- → CH2O (formaldehyde dehydrogenase)CH2O → CH3OH (methanol dehydrogenase)

Photoelectrochemical Reduction

• In anhydrous,aprotic solvent, reduction potential of CO2 to CO2

•- is -2.2V vs NHE!• Large kinetic overvoltage• Yield of photochemical conversion is low• Proton assisted multielectron reductions

more favorableCO2 + 2H+ + 2e- → HCO2H -0.61VCO2 + 6 H+ + 6e- → CH3OH + H2O -0.38V

References• Cotton,F.A.; Wilkinson, G.; Murillo,C.A.; Bochmann, M.; Advanced Inorganic

Chemistry 6th ed.; Wiley-Interscience: New York, 1999.• Greenwood, N.N.; Earnshaw, A.; Chemistry of the Elements; Pergamon: New York,

1984.• Aresta, M. Carbon Dioxide Reduction and Uses as a Chemical Feedstock. In

Activation of Small Molecules: Organometallic and Bioinorganic Perspectives; Tolman, W.B., Ed.; Wiley-VCH: Weinheim, 2006.

• Stumm, W.; Morgan, J.J.; Aquatic Chemistry: Chemical Equilibria and Rates in Natural Waters; Wiley: New York, 1996.

• Ballou, R.W. Carbon Dioxide. In Kirk-Othmer Encyclopedia of Chemical Technology; Wiley: New York, 2005; Vol. 4, pp 725-742.

• Tommasi, I.; Sorrentino, F.; Utilization of 1,3-dialkylimidazolium-2-carboxylates as CO2-carriers in the presence of Na+ and K+: Application in the Synthesis of Carboxylates, Monomethylcarbonate Anions and Halogen-free Ionic Liquids. Tetrahedron Letters 2005, 46, 2141-2145.

• Darrensbourg, D.J.; Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2. Chem. Rev.2007, 107, 2388-2410.

• Leitner, W. Carbon Dioxide as a Raw Material: The Synthesis of Formic Acid and Its Derivatives from CO2. Angewante Chemie International Edition in English 1995, 34, 2207-2221.